Synthesis and Photophysical Properties of a Covalently Linked Porphyrin Chromophore–Ru(II) Water Oxidation Catalyst Assembly on SnO2 Electrodes

We describe here the preparation and surface photophysical properties of a covalently linked, chromophore-catalyst assembly between a phenyl phosphonate-derivatized pentafluorophenyl-substituted porphyrin and the water oxidation catalyst, [RuII(terpyridine)­(2-benzimidazolylpyridine)­(OH2)]2+, in a derivatized assembly of porph-RuII–OH2 2+. The results of nanosecond transient absorption measurements on nanoparticle SnO2 electrodes in aqueous acetate buffer at pH 4.7 are consistent with rapid electron injection into SnO2 with transfer of oxidative equivalents to the assembly. Electron transfer from the singlet excited state of the porphyrin to the conduction band of the electrode, SnO2(e–)|-porph+-RuII–OH2 2+, is favored as the porphyrin singlet excited state lies 0.44 eV above the SnO2 conduction band edge. Electron injection is rapid (⟨τinj⟩ < 10–8 s), and occurs with high efficiency. Based on measured redox potentials, following excitation and injection, intra-assembly oxidation of the catalyst, -porph+-RuII–OH2 2+ → -porph-RuIII–OH2+ + H+, is favored in the transient equilibrium state by 0.62 eV at pH 4.7. However, immediately after the flash, a distribution exists at the surface between isomers with SnO2(e–)|-porph+-RuII–OH2 2+ undergoing back electron transfer to the surface with an average lifetime of ⟨τ1⟩ ∼ 10–7 s and a slower component for back electron transfer from SnO2(e–)|-porph-RuIII–OH2+ with ⟨τ2⟩ ∼ 4 × 10–5 s.